ANOTHER HP-25. Doens´t work


I have an hp-25 which don´t work as it should. Is swithes on and lets you keying numbers. But when you press ENTER, +, -,*,/, clx and some other keys the screen shows “00000000”.
In PRG mode it let you see all the program steps showing “40” some of them, and “13 00” others but it doesn´t let you change anything.

Does anybody have any experience repairing these calcs

Thanks in advance



Hi, Jon;

I hanve an HP25C that's exactly like this: accepts any code you enter in PRGM mode, accepts numbers when keyed in, if you try and yours is like mine, you can even STOre and ReCalL them; but if you try anything else, like ENTER, CLX, EEX, CHS... clears the display for about one second and shows [ 00000000 ], right?

At the moment you pres any key except digits, [STO] n, [RCL] n (did I miss anyone else?) ROM is read for instructions to execute the routine associated with that keycode. All of ROM is searched (blank display) and as the last position is reached, whatever is found is shown: a bunch of zeroes.

I think a copy of the ROM contents may be created from an original ROM, but I have no idea about how to access the HP25 ROM chip and which one can be used to replaced it. If so, copying an HP25 ROM to use in an HP25 is no copyright violation, it's maintenance.

I have a working HP25; if you find a way to copy its ROM, let us know. I also have a not-working HP25C, and I have no way to copy its ROM contents.

Sorry I did not help the way you need.

Luiz C. Vieira - Brazil


Thanks for the information Luiz.

I have tried replacing all the capacitors in the circuit because I have repaired some spices in this way. Nothing changes. So, you must be right.

If I can do anything I will make you know


Hello, Jon;

I remember that some months ago I posted a message entitled "HP25 Scary Moments" where I described the "loss of memory"in my HP25 (not the "C" one). It suddenly was no longer able to record nothing, neither programs, nor data in numbered registers.

I decided to do something and resoldered all solder points in the mainboard, not only the ones around the chips. Well, wherever was the problem, it was solved with a few soldering activity.

maybe, just maybe, it will help you. If your HP25 was working fine and suddenly it stopped working (even if it was sored somewhere and you decided to allow it to see the sunshine again) chances are you have a bad contact or a cracked copper trail somewhere. If you have the time, get the multimeter and measure continuity on copper trails around the three chips closer to the batteries' conectors.

Good luck and success.

Luiz C. Vieira - Brazil


Displays of 0000000 are almost always caused by a bad ACT chip (the big 22 pin one at the bottom of the circuit board).

The indication of 13 00 or 40 in the program registers is an indication of a bad RAM chip (the chip just above the ACT chip. Some machines have two RAM chips (above and to the right of the ACT). I suspect that you cannot store and recall numbers either.


Hello, David;

It's always time to read and get precise information, and I always try to do it.

I read about ACT chips at this forum, about two years ago; I was not aware of some particular internal structures and from then on, I try to keep track with all, new information. I used to think when ACT chip was bad, calculator's overall operation was compromised, meaning it would act erratically. Based on these assumptions, I thought the ACT chip would carry the basic LED display control, KBD scan activity and number entering. ROM would be changed for each particular model. For example, HP27 and HP25 share everything except ROM; is it correct?

I thought if display is correct, number entering is fine but functions and/or memory is not O.K., then ACT chip is fine and ROM/RAM is failure. When my own HP25 was no longer able to hold numbers and programs (register would hold 0.00 and each program line was 13 00, whatever you stored there) and it was solved by resoldreing terminals, I was sure my "theory" was correct.

But I want both: helping with correct information and keep my own "babes" working fine. If I have given information that does not correspond to actual facts and as a consequence I may cause dammage to calculators that do not belong to me (and mine too), please, let me know for both reasons mentioned above.

I want to, please, call all contributors in here for correcting me anytime it's needed.

Thank you very much.

Luiz C. Vieira - Brazil


Luiz writes: For example, HP27 and HP25 share everything except ROM; is it correct?

I don't think so; most of the Woodstocks use PMOS circuitry but the HP-27 is unique in that it uses NMOS. This would imply that HP-27 chips are not interchangable with any other machine. I would imagine that the 27's power supply would also produce different voltages; has anyone confirmed this?

By the way, does anyone know what chip technology was used for the Spices? Perhaps someone could compare the power supply voltages with earlier machines.

- Michael


Hello, Michael;

I wonder what was I trying to use as an example... What I was supposed to mention is that if the ACT main IC is used in so many structures, like the HP25 and the HP27, and they have so diverse features, that this means they share the same structure but ROM contents and RAM organization; is it correct? I thought about one thing and wrote another completely different. I felt so embarrassed I didn't remember what I was actually going to write.

Thank you for noticing my mistake.

Luiz C. Vieira - Brazil


All the woodstock series (except HP27) ACT chips are compatible, even though they have different part numbers. You can freely swap them around... even among the non-C and C machines. Also includes the HP67 and HP97.

It is possible that a bad chip on the internal bus can make other chips appear bad, but certain failures seem to be always (like over 99% of them time) due to specific chips.

If you can't store or recall to memory or the program is all 13 00 or 40's then the RAM chip is bad. Some machines have two RAM chips, so it may be possible that only part of the memory is bad. Some RAM chips (especially the 25C) may me able to be made to work by grounding certain "unused" pins. See Katie Wass's piece in the "articles" section.

A display of 00000000 is "always" a bad ACT chip. Usually it is a hard failure, but sometime it may be an intermittent problem (that will soon go permanent).


Sorry, I am new in this "woodstock world".
Which one is the ACT chip?. I have a working HP-21. Can I use its ACT chip for the HP-25?.
I have both problems (RAM nok and display showing 0000000). What happens if I get a good ACT chip but I can´t repair the RAM chip?.

Thanks to everybody



Hi Jon,

Are you really sure you want to sacrifice a working calculator to fix a broken one? Seems kind of drastic.

Anyway, it seems to me that if the ACT isn't working then that could also be producing the bad RAM symptom. If you must do the repair, try replacing the ACT and see if the RAM problem clears up at the same time. (If the RAM still doesn't work at this point, you're stuck -- you'll have to find a RAM chip from another machine.)

- Michael


Hello, Michael;

I tried to find my notes about the ACT (Arithmetic-Clock-Timer? Is it correct?) chip location and I did not find them. I have in my mind the following location: under the LED display, right chip.

Is it correct?

Thank you.

Luiz C. Vieira - Brazil


The ACT (Arithmetic, Control and Timing) chip is the 22 pin, 0.4" wide chip near the bottom (or front). It is like the CPU in the system.


Hello, Ellis;

thank you. I was reading the posts in this thread again and I found the one where David describes its position... a "senior moment" of mine?

Is the ACT chip in the HP21 also compatible? I had an HP21 in hands somtime ago, and I remember I saw only three IC's: the two used to drive the display and the main IC. Is this an ACT chip as well?

I'm curious because if I am correct, the ACT chip must do everyhting, including RAM and ROM functionality. If so, all scientific functions, stack manipulation and basic 1 register memory are already there. If I am correct, the built-in scientific functions are not used in the HP22.

What am I missing?

Sorry, guys, I sometimes feel as having a gap of understanding when I reason about some isolated facts. Thanks for any additional information.

Luiz C. Vieira - Brazil

(Hey, Ellis; I'm preparing a pack with some material to send you, related to your e-mail, O.K.?)


Luiz, did you see me this morning, asking for a manual that is on the Museum CD-ROMs?! See if you can get a TV station (or cable or satellite provider) in your area to carry "The Red Green Show", it helps us "older" guys cope with the ravages of time!

Also, we sometimes overlook the many resources of the Museum. It's kind of like Doctor Who's Tardis - it has many rooms that are seldom entered. Before I wrote my message, I went to the page on Woodstock technology and made sure what ACT stood for! The ACT does contain the stack registers and one user storage register. Since the chip on the left under the display contains ROM as well as its display function, maybe it contains all the code needed for the HP21? I only have the 25 and it has 1 (or 2) RAM chip(s) and another ROM down near the ACT. If I remove the lower ROM and the RAM, will I have a 21 or is the upper ROM specific to the 25? BTW, after my recent study of HP part number prefixes, I can recognize the general function of the chips at a glance - the ACT and the upper right chip are logic (1820 prefix) and the ROMs and RAM are memory (1818 prefix).



Idon´t really like breaking the HP-21 but I use these machines fur fun. It is fantastic when they come back to life. If I can´t repair it I will place the chip back to the HP-21.
Anyway it is not still clear for me if HP-21 and HP-25 have the same ACT chip


Yes, the HP21 ACT chip will work fine in any woodstock machine except the HP27. It even works in the HP67 and HP97. It is the big fat (0.4 inch wide) chip at the bottom of the circuit board near the battery contacts.

They can be fairly sensitive to a heavy handed removal. Best to use a static controlled vacuum desoldering station.

If the RAM chip is bad, the HP25 will work fine as a simple calculator, but you will not be able to use memory registers or enter programs or the summation function. (Best way to check out an HP25 in two keystrokes is power on and press E+ E+. If you don't see 2.0, the machine is bad).

The HP21 does not have a RAM chip. Also the machines have ROM code in the smaller of the two display driver chips (the ROM0/anode driver). This chip cannot be swapped between different machine types.

Possibly Related Threads...
Thread Author Replies Views Last Post
  Strange HP 25 Problem (Repair) Onur Ilkorur 5 557 12-06-2013, 05:13 PM
Last Post: Onur Ilkorur
  HP-25 Wayne Selway 1 252 11-22-2013, 12:58 PM
Last Post: Michael de Estrada
  emu48 - copy stack doesn't work (as expected) Thomas Radtke 2 468 11-11-2013, 02:19 PM
Last Post: Thomas Radtke
  Another Prime mystery - templates don't work anymore Michael de Estrada 2 355 11-02-2013, 11:34 AM
Last Post: Michael de Estrada
  Interesting Base Conversions - Porting a 1975 HP 25 Program to the HP 35S Eddie W. Shore 1 275 10-13-2013, 07:49 PM
Last Post: BruceH
  Does the HP Prime software only work with Win 7/ 8 ? Michael de Estrada 3 409 10-12-2013, 02:52 PM
Last Post: John Ioannidis
  Back to School/Work/Other Calculator Poll Eddie W. Shore 63 4,442 09-12-2013, 06:03 AM
Last Post: bhtooefr
  HP-25/25C Potential Matt Agajanian 14 951 07-17-2013, 02:57 AM
Last Post: Matt Agajanian
  How I use my HP50G at work. David Bengtson 7 519 05-06-2013, 02:57 PM
Last Post: Ingo
  Does IR printing not work on your converted WP34s? Then read on... Harald 3 402 04-04-2013, 05:46 PM
Last Post: Harald

Forum Jump: